As the clock struck 8:30 p.m. local time across the world Saturday, a silent but powerful sound could be heard: millions switched off their lights in observance of the sixth annual Earth Hour.

The initiative aims to draw attention to energy use and climate change by encouraging homes and businesses to go dark for one hour. While most visible through darkened national landmarks like the Eiffel Tower and Empire State Building, the Earth Hour commitments span the globe and touch into tiny towns and villages on six continents.

This year, its organizers estimate that Earth Hour 2012 is taking place in a record 150 countries and territories across 6494 towns and cities. In 2011, 1.8 billion people were in a country or city participating in Earth Hour. Considering Earth Hour’s modest beginnings in Sydney, Australia, in 2007 when 2.2 million people joined in, they’ve seen an unparalleled growth spurt.

The small South Pacific island nation of Samoa was the first to flip the switch as Earth Hour crossed its clocks. Its airport switched off its lights as landmarks Down Under prepared to go dark – including the Sydney Opera House and Harbour Bridge. Those were followed by a part of the Great Wall of China, the Brandenberg Gate in Berlin, and dozens of other attractions across all time zones. Earth Hour rallied some new participants this year, too: Libya’s Martyrs’ Square, the hub of the nation’s revolution last year, took part in the ceremony of darkness. Algeria, Bhutan and French Guinea are also participating for the first time this year.

With such a terrestrial name, it’s stunning to hear that Earth Hour has indeed reached beyond Earth. Dutch astronaut André Kuipers is said to be following the global switch-off from aboard the International Space Station – 240 miles (400 km) above our planet. Though Kuipers doesn’t appear to blogging or photographing the momentarily darkened earth, as promised. The European Space Agency astronaut is also an ambassador to WWF, the global environmental group that organizes the event. The groups hope that Kuipers’ photographs will show a marked decrease in light emanating from the Earth as each time zone switches off their lights. “Earth Hour 2012 is a celebration of people power; the world’s largest mass event in support of the planet,” WWF official Dermot O’Gorman said in Sydney before the city turned off its lights.

To be sure, by 9:30 p.m. local time all cities are back to full power, but WWF hopes that the message carries on into future hours.